1. Field of the Invention
The invention relates to a system and method for expanding a laser beam without expanding its spatial coherence.
2. Background Art
In lithography, or other environments (e.g., holography), expansion of an excimer laser beam or deep UV (DUV) excimer laser beam is necessary because an illumination system field is typically much bigger than the laser beam. Typically, laser beams are 10 mm×10 mm or 5 mm×20 mm, while an illumination field may be 120 mm×25 mm. Although the laser beam is described as having a rectangular or square cross-section, various cross-sections of light can be used. Generally, lithography devices use an arrangement consisting of one reflector and one partial reflector (or beam splitter) to preliminarily expand the laser beam in an optical multiplexer before expanding the preliminarily expanded beam further in other parts of the lithography tool. Unfortunately, expansion with typical optical devices (lenses, prisms) increases the spatial coherence of the laser and creates a speckle problem. Therefore, other optical devices can be used. The drawback of using the reflector/beam splitter arrangement is that it requires a complicated design of a “staircase” partial reflector, which consists of patches of coatings having a stepwise change in reflectivity based on predetermined parameters. This arrangement requires an exact match of the size and position of the laser beam and the “staircase” patch pattern. Also, a practical implementation of the “staircase” partial reflector leads to uncoated areas between the patches and the expanded beam, which results in a “zebra” pattern with dark areas cutting through bright areas of a beam cross section. Further, excimer lasers have a tendency to change the beam size and divergence over the time.
Therefore, a system and method for expanding an emitted light from a laser without changing spatial coherence of the light, without producing speckle patterns, and that eliminates the requirement for the “staircase” partial reflectors is needed.